Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communications systems. In particular, wide area communications networks employ long-haul transmission links using single mode optical fibres for the transmission of digital information at very high bit rates, using one or more optical carriers, or wavelengths, over each fibre. The distances over which data may be transmitted in single-mode optical fibres before some form of regeneration is required is limited by optical attenuation and pulse dispersion. The advent of practical optical amplifiers has substantially eliminated the loss limitation, particularly for systems operating in the third optical communications window at wavelengths around 1550 nm, in which erbium-doped fibre amplifiers are applicable. However, dispersion processes, including chromatic dispersion and polarisation mode dispersion (PMD), which lead to pulse broadening, remain a significant cause of distortion in such systems, which if not managed or compensated can severely limit the reach of optical transmission spans.
The problem presented by chromatic dispersion increases rapidly as the bit rate of optical data channels is increased. This is because, on the one hand, increasing the bit rate results in an increase in the spectral width of transmitted channels, and hence increased pulse broadening as a result of chromatic dispersion. On the other hand, increasing the bit rate also results in a reduction in the bit period ie the time interval between consecutive bits. In wavelength division multiplexed digital transmission systems in particular, it is not practical to reduce pulse broadening by employing optical fibre having a low or zero dispersion value near the transmission wavelength, because a low first order dispersion value is associated with increased distortion due to non-linear processes.
The impact of PMD also increases at higher data rates, again due to the reduced bit period, and to some extent also due to spectral broadening, particularly when higher-order PMD processes are considered.
Accordingly, methods and apparatus that are able to compensate for the effects of dispersion within optical fibre transmission spans have taken on increasing importance in high capacity optical transmission systems.
Known methods of compensating for the effect of chromatic dispersion include pre-chirping of transmission lasers, mid-span optical phase conjugation of data channels, the use of chirped-fibre Bragg gratings having dispersion characteristics opposed to those of the optical fibre transmission span, and the use of highly dispersive dispersion compensating fibre.
However, these methods are not without their drawbacks. In particular, all operate substantially within the optical domain, and typically use components providing a fixed amount of dispersion compensation. Accordingly, these components must be designed and/or configured to match the characteristics of the specific transmission spans in which they are installed, and they are not readily dynamically adaptable for use in different transmission spans, or in systems exhibiting varying total chromatic dispersion.
On the other hand, it is relatively straightforward to design and construct electronic systems, including analogue and/or digital systems, that may include software components, and that are highly adaptive to changing requirements. In particular, adaptive electronics have been extensively applied in radio frequency (RF) communications systems, including both wireless and wireline systems which are able to dynamically compensate or equalise channel characteristics. There has therefore been interest in recent times in devising methods and apparatus enabling more sophisticated electronic processing techniques to be used to mitigate the effects of dispersion in optical transmission spans. Electronic dispersion compensation may be used, for example, to upgrade existing transmission links without replacing or augmenting installed optical plant. Furthermore, electronic dispersion compensators could be designed to adaptively respond to dynamic changes in total dispersion, such as may occur in systems employing all optical switching and transmission technologies.
A significant obstacle to the implementation of electronic dispersion compensation within optical transmission systems is that most high-bandwidth optical systems employ intensity modulation at the transmitter, in combination with direct detection at the receiver. Intensity modulation results in optical signals having two frequency sidebands disposed about a central optical carrier frequency, and direct detection of such signals results in a loss of the optical phase information which is required to enable the effects of dispersion generally, and chromatic dispersion in particular, to be compensated. Accordingly, methods of performing electronic dispersion compensation at the receiving end have been proposed which involve the transmission of signals that do not have the conventional double-side band frequency spectrum which results from intensity modulation. In particular, electronic dispersion compensation methods using optical single sideband (OSSB) or vestigial sideband (VSB) have been proposed, in which the optical phase information is translated directly into electrical phase information at the optical receiver. Additionally, it has been proposed to generate such signals in which a number of RF sub-carriers are multiplexed within the transmitted single optical sideband. Because each such sub-carrier may have a significantly narrower bandwidth than the overall bandwidth of the transmitted optical signal, an increased tolerance to dispersion may be achieved.
However, previously proposed electronic dispersion compensation techniques based on the use of OSSB transmission and/or RF sub-carrier multiplexing suffer from a number of remaining limitations. Firstly, the quality and cost of the RF components, including RF filters, mixers and so forth, limit the number of RF sub-carriers that may be employed, and the spectral efficiency of the sub-carrier multiplexing. Furthermore, similar limitations and/or costs are imposed at the receiver, where the RF sub-carriers must be demultiplexed and compensated, or equalised, independently. Additionally, the systems proposed to date have exhibited comparatively poor optical power efficiency.
There is therefore an ongoing need for improved methods and apparatus for the generation and transmission of optical signals which enable effective dispersion compensation to be performed in the electronic domain, while mitigating the aforementioned disadvantages of known methods and systems.